Optical systems for the alignment of the main operative axis of an apparatus, such as a radar, are used to achieve angular alignment, or to measure misalignment errors, between the operative axis of the apparatus and the optical axis of the sighting device which is usually in the form of a small telescope having a lens system whose optical axis extends essentially parallel to the operative axis of the apparatus to which it is attached. Usually, the apparatus and the optical sighting system are supported on a common pedestal to achieve simultaneous rotation in azimuth and in elevation angle. Optical sighting is convenient where there are well defined visible targets whose relative location is accurately known. For example, in a case where it is necessary to align the azimuth measuring pick-off of a radar to true north, there may be a distinctive optical target such as a church steeple on the horizon whose bearing is known. Angular alignment of the north pick-off can be checked by directing the main axis of the apparatus toward the church steeple and checking its alignment with the optical sighting device, assuming that the optical sighting device has an axis which is set precisely parallel to the main axis of the radar, which comprises the RF axis of the antenna, and assuming the proper allowance is made for parallax between the main and optical axes.
In a typical radar antenna system which is not covered by a Radome, the optical axis of the sighting device presents little problem because the sighting device can be mounted in close proximity to the radar antenna, it being common practice to have the optical sighting line extend through a hole in the antenna reflector. In such a case, the mechanical alignment between the antenna and the optical axis of the sighting device can be maintained to very close tolerance. However, many high precision tracking radar systems have their antennas covered by a Radome in order to reduce the detrimental effects of sun heating and wind on the positioning of the antenna. Since in most cases the Radome cannot be made sufficiently transparent visually that the optical sighting device can see through it, it is accordingly necessary to offset the optical sighting device to one side of the antenna on an arm which extends through the Radome along the elevational axis of the antenna mounting, whereby elevational changes in the radar position will rotate the arm. The Radome frequently is mounted so that it rotates in azimuth with the antenna pedestal but does not rotate in elevation. The sighting device itself according to the prior art has a reticle incorporated into it which produces a substantially 1:1 correspondence between the angular motion of the optical sighting device and that of the reticle. However, a problem occurs in the prior art systems when the coupling arm which supports the optical device and the reticle flexes or vibrates, thereby causing angular misalignment between the optical sighting device and the main axis of apparatus being aimed at the target. Any mechanical motion of the optical sighting device will of course result in a faulty indication of alignment of the main operative axis of the apparatus, and the greater the offset due to length of the optical device coupling arm, the greater the error tends to be. This becomes even more serious when it is remembered that the optical sighting device necessarily has considerable mass which is concentrated at the outer end of the supporting arm, and this condition is still further aggravated when a television-type of sighting system is employed since the television camera must be included in the mass of the optical sighting device. The greater the length of the coupling arm, and the greater the mass of the optical device at its outer end, the heavier and more rugged the arm must be in order to minimize misalignment errors, and this additional weight is in many cases highly undesirable in a radar antenna system, or for that matter in other mounting systems to which the present invention may be applied as recited under the "Field of Invention" heading of this disclosure.
Quantitatively, in a high precision tracking radar system the angular position of a target being tracked can be determined in many cases with an over-all error of the order of 0.1 milliradian. This over-all error is the aggregate of individual contributing errors most of which are themselves substantially less than 0.1 milliradian. A typical specification for a permissible contributing error limit may be of the order of 0.03 milliradian. In many precision radar tracking systems, the azimuth accuracy tends to be better than the elevational accuracy, especially for low elevation angles. When a radar is tracking an aircraft at long range, such a low elevational angle is the usual case because of the limited altitude at which the aircraft can fly as compared with the range at which the radar can see the aircraft. The accuracy with which the aircraft altitude can be determined by means of radar at such low elevation angles is necessarily limited to the errors inherent in estimating the amount of bending effect of the RF beam caused by the troposphere, and by multipath reflections off of the earth's surface. Referring for the moment only to troposphere effects ("Radar System Analysis" by Barton, Prentiss-Hall 1964, Section 15.3) for example a ground radar system which is tracking an aircraft flying at an altitude of 15 kilometers and at a range of 300 kilometers experiences elevation bending of the radar RF beam of approximately 4 milliradians. The exact amount of bending depends on atmospheric parameters which are difficult to measure with great accuracy. As a result, often only approximate correctios can be made of the order of several tenths of a milliradian. In contrast, the azimuth or elevation angle accuracy of a high precision radar as mentioned above can be typically of the order of 0.1 milliradian in the absence of such atmospheric effects. Since the tropospheric and multipath bending effects are more pronounced in elevation, the azimuth data is usually considered better for long range aircraft targets. In air traffic control systems, the elevation of a cooperative aircraft can be determined by the aircraft's own instruments, such data being translated back to the ground radar. As a result of the foregoing, it is frequently advantageous to utilize means for maximizing azimuth data accuracy as compared with elevation data accuracy. This can be implemented by utilizing one of the further improvements taught below; namely, the use of a pentaprism oriented to minimize azimuth error.